1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, which is used in developing electrostatic latent images with a developer in electrophotography or in an electrostatic recording process, etc., to a method for producing it, and also to an electrostatic image developer, a method for forming an image and an image forming apparatus.
2. Description of the Related Art
Visualization of image information via electrostatic images in electrophotography or the like is now utilized in various fields. In electrophotography, electrostatic latent images are formed on a photoreceptor through static charging and exposure to light, then they are developed with a toner-containing developer, and the resulting toner images are transferred and fixed on recording media, on which the images are thus visualized.
The developer to be used therein includes a two-component developer that contains a toner and a carrier, and a mono-component developer of a magnetic or non-magnetic toner alone. For producing the toner, generally employed is a kneading and grinding method that includes melting and kneading a thermoplastic resin along with a pigment, a charge control agent and a release agent such as wax, cooling the resulting mixture, then grinding it, and classifying the resulting particles. To the toner, optionally added are inorganic or organic particles, which adhere to the surfaces of the toner particles to thereby improve the flowability and the cleanability of the toner. The method is effective for producing high-quality toners, but involves some problems such as those mentioned below.
In such an ordinary kneading and grinding method, the morphology and the surface structure of the toner particles produced could not be controlled, and they delicately vary depending on the grindability of the starting materials used and on the condition employed in the grinding step. In the method, therefore, it is difficult to obtain toner particles having a specifically defined morphology and a specifically defined surface structure. In addition, in the kneading and grinding method, the starting materials to be used are limited. Concretely, the resin colorant dispersion to form a toner must be brittle so that it can be well ground in an economic device. However, if the resin colorant dispersion is too brittle, the toner containing it will release fine powder in a development unit, when having received mechanical shear force in the unit, and, as the case may be, the toner morphology will be often changed. This will have some negative influences on the function of developers. For example, in a two-component developer, fine powder of the toner will adhere to and solidify on the surfaces of carrier particles to thereby much deteriorate the chargeability of the developer; and in a mono-component developer, the particle size distribution of toner particles will be broadened, and, as a result, the toner will scatter and its morphology will change to worsen its ability to develop images, whereby the quality of the images developed will be lowered. In case where a large quantity of a release agent such as wax is added to prepare a toner, the release agent is often exposed out on the surfaces of the toner particles, depending on the type of the thermoplastic resin combined with it, and this is problematic. In particular, in a combination of a resin which contains an increased amount of a high-molecular component so as to increase its elasticity and which is therefore difficult to grind, and a brittle wax such as polyethylene, polyethylene will be readily exposed out on the surfaces of toner particles. This will be favorable in point of the releasability in toner fixation and of the cleanability of non-transferred toner from photoreceptors, but the polyethylene having been exposed out on the toner surfaces will readily move owing to external force applied thereto, and will soil development rollers and photoreceptors and or will contaminate carrier particles, whereby the reliability in image formation will be lowered.
For amorphous toner, its flowability could not be increased to a satisfactory level even when a flowability improver is added thereto. In that case, fine particles existing on the surfaces of the toner particles will move to the recesses of the surfaces owing to the mechanical shear force that may be applied to the toner particles in a duplicator, whereby the flowability of the toner will be time-dependently lowered; or the flowability promoter will be embedded inside the toner particles whereby the developability, the transferability and the cleanability of the toner will be worsened. In case where the toner recovered in a cleaning zone in a duplicator is returned to the development unit therein, the quality of the images formed will be further worsened. If the amount of the flowability promoter in the toner is increased so as to solve the problems, peppers will appear on photoreceptors, and the promoter particles will much scatter.
Recently, some methods have been proposed for controlling the morphology and the surface structure of toner particles. For example, one method proposed in Japanese Patent Laid-Open Nos. 282752/1988 and 250439/1994 is for producing toner through emulsion polymerization combined with condensation. This includes separately preparing a dispersion of resin particles formed through emulsion polymerization, and a dispersion of a colorant in a solvent, mixing them to form condensed masses of which the size corresponds to the size of the toner particles to be produced, and heating and melting them to form toner particles. According to the method, the toner morphology can be controlled in some degree and the chargeability and the durability of the toner produced can be improved. However, since the inner structure of the toner particles produced therein is nearly uniform, the method is still problematic in that the releasability of the sheet on which toner images are fixed is not good and the transparency in OHP output is not stable.
As so mentioned hereinabove, toner must stably exhibit its capability even under various types of mechanical stress in electrophotography. For this, too much exposure of a release agent out of the surfaces of toner particles must be prevented, the surface hardness of toner particles must be increased not detracting from the fixability of toner, the mechanical strength of toner must be increased, and both the chargeability and the fixability of toner must be ensured.
High-quality images are much desired these days. In particular, high-precision color images are desired, for which the particle size of toner particles is reduced more and more. However, if the size of conventional toner particles having an ordinary particle size distribution is reduced, fine toner powder will increase. Such fine toner powder is problematic in that it contaminates carrier particles and soils photoreceptors, and it scatters. For these reasons, it has heretofore been difficult to realize high image quality and high reliability even though the size of toner particles is reduced. To solve the problem, it is important to realize a sharp particle size distribution of toner particles and to reduce the particle size thereof.
In recent digital full-color duplicators and printers, a color image original is separated into B (blue), R (red) and G (green) through individual color filters, then latent images of dots each having a diameter of from 20 to 70 xcexcm are formed, corresponding to the original, and they are developed with toners of Y (yellow), M (magenta), C (cyan) and Bk (black) through subtractive color mixture. As compared with conventional monochromatic image-forming machines, a larger quantity of toners must be transferred in such full-color image-forming machines, and in addition, toners corresponding to small-size dots of latent images must be used therein. To that effect, it is more important that the toners for latent images of such small-size dots meet the requirements of uniform chargeability, durability, mechanical strength, and sharp particle size distribution. Considering the recent tendency in the art toward high-speed energy-saving image-forming machines, toners are desired to be fixable at lower temperatures. In view of these, the condensation and melting method is especially favorable for producing small-size toner particles having a sharp particle size distribution.
In full-color image-forming machines, plenty of toners must be well mixed, and it is indispensable to increase the color reproducibility and OHP transparency of the toners.
In general, toners contain a release agent such as polyolefin wax, which is to prevent low-temperature offset in toner fixation. In addition, a small amount of silicone oil is uniformly applied to fuser rollers to thereby improve the high-temperature offset resistance in image formation. As a result, silicone oil often stains the media with images outputted thereon, and the media are sticky and give an unpleasant feel to users. Therefore, applying silicone oil to fuser rollers is undesirable.
To solve the problems, a toner for oilless fixation has been proposed in Japanese Patent Laid-Open No. 61239/1993. This contains a large quantity of a release agent. As containing such a large quantity of a release agent, the releasability of the toner is improved in some degree. However, since the release agent is miscible with the binder resin therein, it could not be uniformly and stably released from the fusing toner, and, as a result, the toner release stability is still unsatisfactory. In addition, since the cohesive force of the binder resin depends on the molecular weight and the glass transition point thereof, it is difficult to directly control the cobwebbing property and the cohesiveness of the fused toner. In addition, some components released from the release agent often interfere with the chargeability of the toner.
To solve the problems, some methods have been proposed. For example, a high-molecular component is added to a binder resin to thereby improve the toughness of the binder resin, as in Japanese Patent Laid-Open Nos. 69666/1992 and 258481/1997; or a binder resin is chemically crosslinked to enhance its toughness, thereby to reduce the cobwebbing property of toner at temperatures at which the toner fuses for fixation and to improve the toner releasability in oilless fixation, as in Japanese Patent Laid-Open Nos. 218460/1984 and 218459/1984.
When a crosslinking agent is added to a binder resin so that the binder resin is thereby crosslinked, as in Japanese Patent Laid-Open Nos. 218460/1984 and 218459/1984, then the stickiness of the toner that contains the crosslinked binder resin, or that is, the cohesiveness thereof in melt will be increased and the toughness of the binder resin itself will be increased, whereby the releasability of the toner in oilless fixation that depends on the processing temperature and on the spreadability of the toner could be improved in some degree. However, even in that method, it is still difficult to improve the surface gloss of the fixed images. In the method, in addition, the folding resistance of the fixed images is poor. When the molecular weight of the crosslinking agent to be added to the binder resin is increased, as in Japanese Patent Laid-Open No. 218460/1984, then the molecular weight of the crosslinked binder resin in the crosslinked region thereof could be increased and therefore the flexibility of fixed images could be improved in some degree. However, it is still difficult to ensure well-balanced elasticity and stickiness of the toner. As a result, it is therefore difficult to satisfy all the requirements of improved toner releasability in oilless fixation, not so much depending on the processing temperature and on the toner spreadability, and improved surface gloss of fixed images and improved OHP transparency. In addition, it is also difficult to control the difference in the surface gloss between different colors of fixed images, while ensuring the expression of the desired surface gloss of fixed images. In particular, when the proposed method is applied to energy-saving fixation units and to high-speed duplicators and printers, satisfactory fixed images could not be obtained.
In Japanese Patent Laid-Open No. 69664/1992, proposed is a method of adding polymer particles or inorganic particles to a toner to thereby improve the high-temperature offset resistance of the toner being fixed. Owing to their filler effect, inorganic particles added to a toner could improve the toughness of the binder resin in the toner while the toner is fused and fixed, whereby the high-temperature offset resistance and the releasability of the toner could be improved. However, the inorganic particles added lower the flowability of the fused toner, and will therefore detract from the low-temperature offset resistance of the toner and the surface gloss of fixed images. In addition, they will often lower the folding resistance of fixed images. Depending on their amount added, the inorganic particles merely increase the stickiness of the fused toner but could not improve the releasability of the fixed toner.
The present invention has been made in view of the above circumstances, and provides a toner for developing an electrostatic image and a method for producing it, and also an electrostatic image developer, a method for forming an image and an image forming apparatus. Free from the problems noted above, the advantages of the toner are as follows: The releasability of the toner does not fluctuate in oilless fixation at any processing temperatures; the toner enables fixation of images with a good surface gloss, and has good fixation characteristics including adhesiveness of fixed images onto fixation sheets, releasability of image-fixed sheets, hot offset resistance, folding resistance of fixed images, surface glossiness of fixed images and OHP transparency; and the difference in the surface gloss between different colors of the fixed images is reduced.
Having solved the problems noted above, an aspect of the present invention provides a toner for developing an electrostatic image including a binder resin, a colorant, a release agent and an inorganic particles. The inorganic particles contain inorganic particles (A) having the mean primary particle size not less than approximately 5 nm and less than approximately 30 nm and inorganic particles (B) having the mean primary particle size not less than approximately 30 nm and less than approximately 200 nm.
According to another aspect of the invention, a method for producing a toner for developing an electrostatic image includes a step of mixing a dispersion of resin particles not more than 1 xcexcm in size, a dispersion of a release agent and a dispersion of inorganic particles, a step of preparing a dispersion of aggregated particles, and a step of heating the resulting dispersion to a temperature not lower than the glass transition point or the melting point of the resin particles to form toner particles. The inorganic particles contain inorganic particles (A) having the mean primary particle size not less than approximately 5 nm and less than approximately 30 nm and inorganic particles (B) having the mean primary particle size not less than approximately 30 nm and less than approximately 200 nm.
According to another aspect of the invention, a method for forming an image includes a step of forming an electrostatic latent image on an electrostatic latent image bearing member, a step of developing the electrostatic latent image with a developer on a developer bearing member to form toner image, a step of transferring the toner image onto a transfer medium, and a step of fixing the toner image on the transfer medium. In this method, the developer contains the above-described toner.
According to another aspect of the invention, an image forming apparatus includes an electrostatic latent image bearing member on which an electrostatic latent image is formed, a developer bearing member for bearing a developer, a development unit in which the electrostatic latent image is developed with the developer on the developer bearing member to form a toner image, a transfer unit in which the developed toner image is transferred onto a transfer medium, and a fuser unit in which the toner image on the transfer medium is fixed.